1. Field of the Invention
This invention relates generally to stepping motors and, more particularly, to a circuit for providing the required drive currents to the coils of a stepping motor.
2. Description of the Prior Art
Solenoid actuated rachet motors have been used for many years in telephone switching equipment. Rachet steppers may be operated by external DC pulses or by integral make and break contacts on the solenoid switch frame. Rachet steppers are also used for remote tuning of radio equipment and many other angular rotary functions where unidirectional drive in adequate.
Besides being limited to unidirectional drive (in the absence of special and complex designs), rachet motors have other disadvantages. The incremental step of a rachet motor is seldom less than ten degrees, and the output torque is limited if coil currents are to be held to a reasonable value. Stepping motors, in contrast, are capable of high resolution bidirectional stepping and high rotary torque.
Numerical control of machine tools is usually implemented by means of feedback servo motors driving lead screws and other positioning controls. Input or control data is generally in binary form and then converted to analog signals for controlling the servo motors. Stepping motors, which may be driven digitally, are available with a wide range of characteristics (e.g. stalling torque of 25 ounce-inches with a drive of 100 ma to a stalling torque of 3,000 ounce-inches with a drive of 3.9 amps) and could therefore be applicable to a variety of machine control systems where highly accurate positioning and/or angular control is necessary.
With present stepping motors operating with the usual chopper driver circuits for high speed applications, audible noise is always created (even when switching occurs at ultrasonic frequencies due to intermodulation between two chopping coils within the motor). This also generates unwanted losses and heat dissipation in the motor and driver circuits. (Chopping happens continuously even with a stalled motor.)